A rapid technological innovation has recently been achieved as to the optical recording medium to and from which signals are recorded and reproduced by the use of light. Numerous attempts have been made particularly to increase recording capacity, working out various element technologies for this purpose.
Among such technologies there are those relating to a disk-shape medium to which information is recorded, that is, a so-called optical disk. The optical disk usually has a configuration wherein a recording medium layer is provided on a substrate having tracks thereon. In this case, by making a light spot trace the tracks, the light spot is caused to accurately follow lines of fine recording marks.
In the case of the optical disk, when tracks are formed on the substrate, address information of the tracks are also marked on the tracks at the same time. Specifically, pits indicating track numbers, sector numbers, etc., are formed on the tracks or between the. tracks, and by reading them the address information can be obtained.
In another method for marking address information, wobbled grooves are used. To be more specific, as shown in FIG. 13, both side walls of a groove 101 provided on an optical disk (they are also side walls of adjacent lands 102) are wobbled in a direction orthogonal to a longitudinal direction of the track (the method using such grooves is hereinafter referred to as both-side-wobble method). A wobble frequency obtained therefrom is modulated by a certain method, and address information is allocated to it. In this case, by setting the wobble frequency higher than frequencies to which the light spot is capable of following, the light spot does not follow the wobble frequency during tracking and wobble frequency components are always added to a tracking signal. Therefore, by detecting the wobble frequency components, the address information can be obtained. Incidentally, as a method for increasing the capacity of the optical disk, a land/groove recording method has been proposed. Conventionally signals are recorded on either the grooves or the lands between the grooves, but in contrast, the above method is for recording information on both the grooves and lands. In this case, it is impossible to accurately record address information to lands 102 wobbled in a manner as shown in FIG. 13, since a width of the land 102 varies with wobbles of the two adjacent grooves 101.
As a method to solve this problem, a method using grooves one of whose side walls is wobbled, that is, a single-side-wobble method, is disclosed by the Japanese Publication for Laid-open Patent Application No.5-314538/1993 (Tokukaihei No.5-314538). The method is applied to an optical disk having grooves 111 and lands 112 thereon wherein only one side wall of each groove 111 is wobbled, a s shown in FIG. 14, so that address information is recorded, using a wobble frequency obtained from the wobble.
However, in the case of the both-side-wobble method and the single-side-wobble method both, every groove (or land) has a wobbled side wall, that is, a portion being tracked always has a wobbled side wall. Therefore, a reproduction signal of information includes wobble frequency components since mixture of wobble frequency components occurs when information signals are recorded or reproduced. As a result, signal quality deteriorates.
Besides, due to the above-mentioned signal mixture, it is necessary to set the wobble frequency in a frequency band different from the frequency band for the. information signal, and hence the wobble frequency is set much lower than the frequency of the information signal. Therefore, the spatial wavelength of the wobble frequency becomes long, and accordingly each domain has to be long so as to express each piece of address information. As a result, making divisions between addresses clear and precise is failed.
This does not signifies much in the case of signals to be continuously transmitted, such as video signals or sound signals. However, in the case where an optical disk is used as a memory of a computing device wherein signal transmission is frequently carried out, signals are grouped into blocks when transmitted, and it is necessary to arrange data by a so-called sector method. In this case, the above matter becomes a serious problem.
Besides, since the width of the grooves or the lands varies in some areas, there arise a problem that the beam spot may have an offset, and besides, a problem that the offset is difficult to correct.
There are more methods for providing address information in the case where the land/groove recording method is used. For example, an exclusive address method and a common address method are reported by "Nikkei Electronics" (Nov. 6, 1995, p. 168).
The former is a method whereby at every sector in both the lands and grooves, a pre-pit is provided exclusively for the sector. However, since it is necessary that the pit width is sufficiently narrower than the track width, the pre-pits cannot be formed by the use of a light beam used for forming the tracks. Thus, this method has a defect of making it difficult to manufacture optical disks.
By the latter, that is, the common address method, as shown in FIG. 15, pit series 123 are provided on borders between grooves 121 and lands 122 so that each pair of adjacent groove 121 and land 122 share pit series 123. In this case, a light spot 124 always tracks off the center of pit series 123 when reproducing the pit, during both the tracking operations with respect to the grooves 121 and lands 122. Therefore, there arises a problem that the signal quality is low.
Further, when the light spot 124 passes the pit series 123, the light spot 124 does not tracks on a center line of the pit series 123, thereby causing a great off-set to be added to the tracking signal. Since the pits are intermittently provided, the tracking signal obtained from the pits is also intermittent, but it comes to a great off-set on the average.
A tracking servo system usually does not respond to the tracking signal itself obtained from the individual pit since the pit frequency is sufficiently higher than a frequency band of the tracking servo system. However, since the pit region is considerably long in the present case, the tracking signal is averaged through out the pit region, thereby becoming a signal having a great off-set, to which the servo system responds. As a result, a signal having great spikes is generated when the light spot passes through the pit region. An actuator of the tracking servo system responds to the signal having spikes in the case where the pit region is considerably long, thereby causing a problem of transient response, which reversely affects the signal recording and reproduction.